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| .TH PTHREAD_RWLOCKATTR_SETKIND_NP 3 2019-03-06 "Linux Programmer's Manual" |
| .SH NAME |
| pthread_rwlockattr_setkind_np, pthread_rwlockattr_getkind_np \- set/get |
| the read-write lock kind of the thread read-write lock attribute object |
| .SH SYNOPSIS |
| .nf |
| .B #include <pthread.h> |
| .PP |
| .BI "int pthread_rwlockattr_setkind_np(pthread_rwlockattr_t *" attr , |
| .BI " int " pref ); |
| .BI "int pthread_rwlockattr_getkind_np(const pthread_rwlockattr_t *" attr , |
| .BI " int *" pref ); |
| .PP |
| Compile and link with \fI\-pthread\fP. |
| .PP |
| .fi |
| .in -4n |
| Feature Test Macro Requirements for glibc (see |
| .BR feature_test_macros (7)): |
| .in |
| .PP |
| .BR pthread_rwlockattr_setkind_np (), |
| .BR pthread_rwlockattr_getkind_np (): |
| .br |
| .RS 4 |
| .ad l |
| _XOPEN_SOURCE\ >=\ 500 || _POSIX_C_SOURCE >= 200809L |
| .RE |
| .ad |
| .SH DESCRIPTION |
| The |
| .BR pthread_rwlockattr_setkind_np () |
| function sets the "lock kind" attribute of the |
| read-write lock attribute object referred to by |
| .I attr |
| to the value specified in |
| .IR pref . |
| The argument |
| .I pref |
| may be set to one of the following: |
| .TP |
| .B PTHREAD_RWLOCK_PREFER_READER_NP |
| This is the default. |
| A thread may hold multiple read locks; that is, read locks are recursive. |
| According to The Single Unix Specification, the behavior is unspecified when a |
| reader tries to place a lock, and there is no write lock but writers are |
| waiting. |
| Giving preference to the reader, as is set by |
| .BR PTHREAD_RWLOCK_PREFER_READER_NP , |
| implies that the reader will receive the requested lock, even if |
| a writer is waiting. |
| As long as there are readers, the writer will be |
| starved. |
| .TP |
| .B PTHREAD_RWLOCK_PREFER_WRITER_NP |
| This is intended as the write lock analog of |
| .BR PTHREAD_RWLOCK_PREFER_READER_NP . |
| This is ignored by glibc because the POSIX requirement to support |
| recursive read locks would cause this option to create trivial |
| deadlocks; instead use |
| .B PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP |
| which ensures the application developer will not take recursive |
| read locks thus avoiding deadlocks. |
| .\" --- |
| .\" Here is the relevant wording: |
| .\" |
| .\" A thread may hold multiple concurrent read locks on rwlock (that is, |
| .\" successfully call the pthread_rwlock_rdlock() function n times). If |
| .\" so, the thread must perform matching unlocks (that is, it must call |
| .\" the pthread_rwlock_unlock() function n times). |
| .\" |
| .\" By making write-priority work correctly, I broke the above requirement, |
| .\" because I had no clue that recursive read locks are permissible. |
| .\" |
| .\" If a thread which holds a read lock tries to acquire another read lock, |
| .\" and now one or more writers is waiting for a write lock, then the algorithm |
| .\" will lead to an obvious deadlock. The reader will be suspended, waiting for |
| .\" the writers to acquire and release the lock, and the writers will be |
| .\" suspended waiting for every existing read lock to be released. |
| .\" --- |
| .\" https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_rwlock_rdlock.html |
| .\" https://sourceware.org/legacy-ml/libc-alpha/2000-01/msg00055.html |
| .\" https://sourceware.org/bugzilla/show_bug.cgi?id=7057 |
| .TP |
| .B PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP |
| Setting the lock kind to this |
| avoids writer starvation as long as any read locking is not done in a |
| recursive fashion. |
| .PP |
| The |
| .BR pthread_rwlockattr_getkind_np () |
| function returns the value of the lock kind attribute of the |
| read-write lock attribute object referred to by |
| .IR attr |
| in the pointer |
| .IR pref . |
| .SH RETURN VALUE |
| On success, these functions return 0. |
| Given valid pointer arguments, |
| .BR pthread_rwlockattr_getkind_np () |
| always succeeds. |
| On error, |
| .BR pthread_rwlockattr_setkind_np () |
| returns a nonzero error number. |
| .SH ERRORS |
| .TP |
| .BR EINVAL |
| .I pref |
| specifies an unsupported value. |
| .SH VERSIONS |
| The |
| .BR pthread_rwlockattr_getkind_np () |
| and |
| .BR pthread_rwlockattr_setkind_np () |
| functions first appeared in glibc 2.1. |
| .SH CONFORMING TO |
| These functions are non-standard GNU extensions; |
| hence the suffix "_np" (nonportable) in the names. |
| .SH SEE ALSO |
| .BR pthreads (7) |